SINGAPORE - Healthcare professionals here have received advisories about a dengue vaccine which could be harmful to people not previously infected by the mosquito-borne virus.

Responding to queries from The Straits Times, the Health Sciences Authority (HSA) said on Wednesday (Dec 6) it had alerted them to the latest findings on the vaccine, Dengvaxia.

Produced by French pharmaceutical giant Sanofi Pasteur, Dengvaxia was found to have a possible detrimental effect when used on people who did not previously have dengue. Those who were vaccinated and later became infected could have "more cases of severe disease", Sanofi said in its alert last month.

Sanofi had said it would ask the authorities in countries where it has been approved for use, to update their information on the vaccine, and to disseminate it to doctors and patients.

When asked, the HSA said there had been one report of a male patient who had developed a mild rash three days after being vaccinated, and who has recovered. It said: "Rash is an expected adverse event after vaccination and has been reported in clinical studies and post market use." The patient's dengue status before vaccination was not reported.

HSA said it had received no other reports of adverse reactions to Dengvaxia.

At least one clinic chain has stopped giving the vaccine to people who do not know if they have contracted dengue before.

Most patients who received the vaccine at Parkway Shenton clinics have had a history of dengue infections, said Dr Edwin Chng, Parkway Shenton's deputy medical director.

However, there is a small minority who do not, but have started on the vaccine series.

Dr Chng said: "Moving forward, Parkway Shenton's doctors will not give the second or third dose of the vaccines and will advise patients to continue to take other precautions against dengue infections such as using insect repellent."

The world's first dengue vaccine has been available in Singapore since March, after the HSA in October 2016 approved its use in patients between 12 and 45 years old.

At that time, the HSA had flagged the potential risk of severe dengue after reviewing Sanofi's data. It informed healthcare professionals and recommended that patients consult their doctors before getting vaccinated. Educational materials and the package insert of Dengvaxia also included information on the risk.

In addition, serology testing to identify previous dengue infection was made available at the major hospitals here.

With the latest development, HSA said it is now working with Sanofi to "strengthen the package insert" to include the warning of an increased risk of hospitalisation for dengue and clinically severe dengue in vaccinated individuals not previously infected by dengue.

However, Sanofi said for those who had been previously infected, the vaccine "provides persistent benefit against dengue fever". People who have contracted dengue before can fall ill with it again, as there are four strains of the virus.

The HSA noted that dengue vaccination is not part of the national immunisation programme here, which means the vaccine is given to individuals only when benefits outweigh the risk.

"The benefit-risk profile continues to be favourable for individuals aged 12 to 45 years with previous dengue infection as supported by clinical studies. The necessary safeguards are in place in Singapore such as the availability of serological testing to determine prior dengue infection," it added.